Generally speaking, there are two methods by which the motorboat impeller is fastened with the motorboat driveshaft. The first method includes a impeller which is provided centrally with a threaded axial hole engageable with a threaded portion located at the tail end of a driveshaft. The second method includes a impeller which is provided centrally with an axial hole having therein a predetermined number of serrated slots which extend along the direction of the axis of the axial hole and which are engageable with splines disposed on the tail end of a driveshaft. The motorboat impeller of the present invention relates to the second fastening method described above.
As shown in FIGS. 1 and 2, a conventional motorboat impeller 10 of the prior art comprises an axial portion 12 and a plurality of blades 14 fastened around the periphery of the axial portion 12 of a cylindrical construction. The axial portion 12 has a rather round and smooth front end and is provided centrally with an axial hole 16 extending therethrough along the direction of the axis thereof. The axial hole 16 is provided on the inner wall thereof with a plurality of retaining slots 18 of an appropriate length and extending rearward from the front end of the axial hole 16 along the direction of the axis of the axial hole 16. A motorboat driveshaft (not shown in the drawings) provided at the tail end thereof with splines is fitted into the front end of the axial hole 16 for driving the impeller 10.
The axial portion 12 and the blades 14 of the motorboat impeller 10 are made integrally. The axial portion 12 is provided centrally with the axial hole 16. The portion between the outer surface of the axial portion 12 and the inner wall surface of the axial hole 16 is rather thick so as to facilitate the construction of the retaining slots 18 in the inner wall of the front end of the axial hole 16 with a cutting tool.
The motorboat impeller 10 of the prior art described above has inherent shortcomings, which are expounded explicitly hereinafter.
It is rather time-consuming to construct the retaining slots 18 in the inner wall of the axial hole 16 with a cutting tool.
It is technically difficult to construct the retaining slots 18 with a cutting tool in such a manner that they are arranged equidistantly and circularly in the inner wall of the front end of the axial hole 16.
The cutting tool used to construct the retaining slots 18 is subjected to abrasion after a prolonged usage thereof. As a result, the retaining slots 18 constructed with such a defective cutting tool are bound to have an incorrect dimension. If the dimension of the retaining slots 18 is smaller than a predetermined value, the driveshaft can not be fitted thereinto. If the dimension of the retaining slots 18 is greater than the predetermined value, the splines of the driveshaft fitted into the axial hole 16 are bound to damage the inner wall of the axial hole 16.
It is impractical and uneconomical to use a rigid material to make the axial portion 12 for preventing the retaining slots 18 from being damaged by the splines of the driveshaft in view of the fact that the axial portion 12 is made integrally with the blades 14 which are not subjected to impact force of the splines of the driveshaft. As a result, it is rather wasteful to use an expensive and rigid material to make the axial portion 12 and the blades 14.
The structural strength of the axial portion 12 may be enhanced by an increase in the thickness of the axial portion 12. However, the increase in the thickness of the axial portion 12 results in the formation of a shoulder surface 121 at the front end of the axial portion 12. The shoulder surface 121 is responsible for the formation of water bubbles, turbulent waves and cavitation at the time when the motorboat impeller 10 of the prior art is driven by the driveshaft to turn at a high speed in the water. In other words, the operation of the prior art motorboat impeller 10 can bring about a severe vibration of the motorboat and a substantial reduction in the thrust of the impeller 10.